One of the most challenging puzzles associated with the management of a cellular radiotelephone communication system is faulty coverage recognition. The next is geographically locating an area within the zone of radio coverage or cell where the problem persists. For example, consider an area within a cellular radiotelephone system that provides poor service due to moderate levels of interference. Typically, this situation is uncovered in one of two ways: customer complaints, or the persistent drive team testing by system operators. Neither of these approaches, however, provides a very timely or comprehensive means of identifying, diagnosing, and curing the problem of faulty coverage.
In the past customer complaints have proven an unreliable means of identifying degraded services. Only a small population of the total subscriber community is in the practice of phoning in complaints. As a results, a considerable number of reportable incidences go unattended. This is an unfortunate phenomenon, for greater subscriber participation in faulty coverage reporting would provide an efficient and comprehensive means of faulty coverage identification due to the increased number of addressable complaints.
The use of drive team testing, while more precise than customer complaints, is a labor intensive process, that is expensive, time consuming, and often incomplete. Drive team testing requires dispatching a mobile unit into the selected zone of coverage to make first hand observation of system conditions. This reactive approach to performance evaluation, however, is extremely short sighted. Considering the average geographic area having radio coverage encompasses several square miles, even regular drive team testing is likely to overlook several problems. Furthermore, if the sought after problem is intermittent, it may totally escape the notice of even the most thorough drive team search. Another major shortcoming is the inability of drive team testing to observe the conditions in multiple zones of radio coverage simultaneously.
In response to these deficiencies, the cellular industry has turned to the aid of electromagnetic coverage prediction tools to assist in the search for holes in the coverage. One such coverage prediction tool is described in an article by D.L. Huff, "AT&T CELLULAR TECHNOLOGY REVIEW," IEEE International Conference on Military Communications, pp 490-494, 1985. According to this article, prediction tools "typically employ a propagation `rule` of signal loss for relative heights of the transmit and receive antennas, the intervening terrain, and other special effects such as urban high-rise buildings, water, foliage, etc." The major drawback to this approach is that the prediction is only as good as the assumptions upon which it is made.
Another approach discussed in the Huff article concerns taking certain "live" power level measurements from active and inactive radio units. This approach, while gathering data on actual system performance, falls severely short of providing a comprehensive means of monitoring and diagnosing the state of a dynamic cellular radiotelephone communication system. Under Huff, signal level measurements are taken at the base site, while channel interference measurements are taken from "radios not in active use on calls." This data is then displayed via histogram representations. It will be appreciated by those skilled in the art, however, that Huff's approach possesses very limited capabilities. First, he is incapable of monitoring interference at the location of a mobile unit because all signal strength measurements are base site specific. Next, Huff can not monitor disruptive levels of interference on active channels during periods of peak use. Huff relates to an analog cellular system, thus the mobile radio must be inactive, "not in active use," in order to detect interference. During periods of peak use, cells commonly experience saturation, thereby leaving no inactive channels. Since interference during periods of peak use is of the greatest concern to system operators, Huff represents a tool incapable of addressing this interest. In addition, Huff is unprepared to provide user friendly displays depicting real-time change in the system coverage. Therefore, in order to perform an evaluative diagnosis of a cellular radiotelephone or other communication system, it is necessary to develop a complete history on actual system performance from the mobile unit perspective and to present that history to the system operator in a format chosen for ease of comprehension.